Polyoxometalate−Cyclodextrin Metal
−Organic Frameworks: From Tunable Intrinsic
Microporosity to Customized Storage Functionality
Item Type Article
Authors Yang, Peng; zhao, Wenli; Shkurenko, Aleksander; Belmabkhout,
Youssef; Eddaoudi, Mohamed; Dong, Xiaochen; Alshareef, Husam
N.; Khashab, Niveen M.
Citation Yang P, zhao W, Shkurenko A, Belmabkhout Y, Eddaoudi M,
et al. (2019) Polyoxometalate−Cyclodextrin Metal−Organic
Frameworks: From Tunable Intrinsic Microporosity to Customized
Storage Functionality. Journal of the American Chemical Society.
Available: http://dx.doi.org/10.1021/jacs.8b11998.
Eprint version Post-print
DOI 10.1021/jacs.8b11998
Publisher American Chemical Society (ACS)
Journal Journal of the American Chemical Society
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Communication
Polyoxometalate-Cyclodextrin Metal-Organic Frameworks: From
Tunable Intrinsic Microporosity to Customized Storage Functionality
Peng Yang, Wenli zhao, Aleksander Shkurenko, Youssef Belmabkhout, Mohamed
Eddaoudi, Xiaochen Dong, Husam N. Alshareef, and Niveen M Khashab
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11998 • Publication Date (Web): 04 Jan 2019
Downloaded from http://pubs.acs.org on January 8, 2019
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Polyoxometalate−Cyclodextrin Metal−Organic Frameworks:
From Tunable Structure to Customized Storage Functionality
Peng Yang,
†
Wenli Zhao,
‡
Aleksander Shkurenko,
§
Youssef Belmabkhout,
§
Mohamed
Eddaoudi,
§
Xiaochen Dong,
‡
Husam N. Alshareef,
∇
and Niveen M. Khashab
†,*
†
Smart Hybrid Materials Research Group (SHMs), Advanced Membranes and Porous Materials Center
(AMPMC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.
‡
School of Physical and Mathematical Sciences, Nanjing Tech University, Nanjing 211800, China.
§
Functional Materials Design, Discovery and Development Research Group (FMD
3
), Advanced Membranes and
Porous Materials Center (AMPMC), King Abdullah University of Science and Technology (KAUST), Thuwal
23955, Saudi Arabia.
∇
Materials Sciences and Engineering, Physical Science and Engineering Division, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955, Saudi Arabia.
Supporting Information Placeholder
ABSTRACT: Self-assembly allows structures to organize
themselves into regular patterns by using local forces to find
the lowest-energy configuration. However, assembling
organic and inorganic building blocks in an ordered
framework is hampered by the difficulties of interfacing two
dissimilar materials. Herein, the ensemble of
polyoxometalates (POMs) and cyclodextrins (CDs) as
molecular building blocks (MBBs) has yielded two
unprecedented POM-CD-MOFs, namely [PW
12
O
40
]
3−
& α-CD
MOF (POT-CD) and [P
10
P
15.5
O
50
]
19−
& γ-CD MOF (POP-CD),
with distinct properties not shared by their isolated parent
MBBs. Markedly, the POT-CD features a nontraditional
enhanced Li storage behavior by virtue of a unique
“amorphization & pulverization” process. This opens the
door to a new generation of hybrid materials with tuned
structures and customized functionalities.
With superior properties like chemical tunability and
controlled functionalities,
1
metal−organic frameworks
(MOFs) figure prominently in significant applications, most
notably gas storage,
2
catalysis,
3
sensing,
4
chemical
separation,
5
and protein delivery.
6
One of the unique features
of MOFs is the modularity of their design and synthesis
employing molecular building blocks (MBBs), with their
points of extension dictating the geometry and conductivity
of the resultant MBBs, to guide their self-assembly.
7
Serving
as symmetrical and functional modules suited for
polymerization, a plethora of MBBs have been used to enrich
the repertoire of MOF chemsitry.
8
Aside from the conventional metal carboxylate clusters,
organic macrocycles especially cyclodextrins (CDs) have
been drawn into the construction of MOFs as MBBs.
9
This
emerging family of CD-MOFs have proved to be a “green”
candidate for various high-end uses, including
petrochemicals refining and drug delivery.
10
On the other
hand, inorganic hosts,
11
such as polyoxometalates (POMs),
have been utilized to build POM-organic frameworks
(POMOFs) covering a spectacular structural and
compositional variety, and a myriad of applications in
fundamental and applied sciences.
12
Identifying discrete organic and inorganic MBBs that
spontaneously self-organize into extended frameworks is key
for the successful practice of reticular chemistry. Recent
advances in the assembly of POMs and CDs has resulted in
supramolecular complexes or oligomers without forming
multidimensional networks.
13
In an effort to integrate the
advantages of both parent species namely, CD-MOFs and
POMOFs in one system, a successful preparation of two
POM-CD-MOFs is herein reported for the first time. The
structural configuration could be manipulated simply via
proper selection of POMs bearing different shapes, sizes, and
compositions, such as the classical polyoxotungstate (POT),
[PW
12
O
40
]
3−
(PW
12
), and the newly-discovered
polyoxopalladate (POP),
14
[P
10
P
15.5
O
50
]
19−
(P
10
Pd
15.5
).
Construction of such hybrid assemblies with well-defined
suprastructures prospects a highly customized platform with
superior properties not shared by their independent
components, MBBs. Interestingly, the unique combination of
the intrinsically porous α-CD with the redox-active PW
12
yielded a multilayered assembly that may function as anode
material in lithium ion battery (LIB) with an exceptional Li
storage behavior.
Co-crystallization of the in situ formed Keggin-type PW
12
with α-CD in KCH
3
COO/CH
3
COOH buffer has resulted in a
three-dimensional (3D) framework, K
2.5
Na
2.5
[(PW
12
O
40
)
(CH
3
COO⊂α-CD)(OH)]·19H
2
O (POT-CD), as characterized
and formulated by X-ray structural analysis. The asymmetric
unit is composed of one crystallographically independent α-
CD and PW
12
bridged by K
+
and Na
+
cations, of which the
coordination spheres are completed by oxygen atoms mainly
from the hydroxy groups of α-CD and terminal oxygens of
PW
12
, as well as water molecules (Figure S1). Such a
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connection leads to sub-layer A and B that hold the same
structural pattern (Figure S2). Their staggered overlap yields
a double layer with a certain offset distance, which serves as
the basic repeating unit and extends along the c axis to give
rise to a 3D framework via sharing the metal sites in between
(Figures 1a and S3). With deeper insight into the double-layer
entity, the α-CD toroids nestled in neighboring sub-layers
adopt opposite directions, whereas the PW
12
moieties rotated
horizontally by 90° (Figure 1b). As a consequence of the
synthetic media, one acetate anion is encapsulated by an α-
CD to fulfill a host−guest assembly, as corroborated by both
X-ray diffraction and NMR techniques (Figures S1 and S4).
Figure 1. (a) Combined polyhedral/ball-and-stick
representation of POT-CD. Color code: WO
6
, green
octahedra; PO
4
, pink tetrahedra; K, turquoise; Na, magenta;
O, red; C, yellow. (b) A schematic representation of the
double-layer repeating unit.
Prompted by the above findings, we intentionally replaced
the POM MBBs from the T
d
symmetric PW
12
with a P
10
Pd
15.5
having a pseudo-D
5h
symmetry.
15
As expected, by the help of
K
+
linkage, the combination of the star-shaped P
10
Pd
15.5
with
γ-CD led to a distinct 3D framework, K
23
H
15
[(P
10
Pd
15.5
O
50
)
2
(γ-
CD)
2
]·120H
2
O (POP-CD). The asymmetric unit consists of
two independent γ-CDs and P
10
Pd
15.5
, whose oxygen functions
are mostly coordinated by K
+
ions (Figure S5). Similar to
POT-CD, the skeleton of POP-CD could be disassembled
into two sheets as well. By contrast, each sub-layer in the
latter is comprised solely of POPs or CDs, respectively, rather
than the mixed of both in the former case (Figure S6). Of
prime interest, the γ-CDs in POP-CD parallelly stack with
each other to pillar the framework, forming open channels
running along the a axis (Figures 2a and S7). Of these, four
kinds of γ-CD toroids according to their different
orientations could be grouped as a repeating unit in every
CD sheet. Such well-ordered motifs interweave with the
adjacent POP layers and direct inversely as compared with
their counterparts in the alternate CD sheets (Figure 2b). The
total potential solvent accessible void volume of POT-CD
and POP-CD was calculated to be 352.7 and 7030.5 Å
3
(7.3%
and 21.4% of the respective unit cell volume) using
PLATON.
16
Unfortunately, due to the instability under high
vacuum upon solvent removal, the BET surface area of the
as-made MOFs could not be accessed. However, supported
by CO
2
adsorption data, we could partially access the
porosity of POT-CD, hence confirming its porous feature
(Figure S8).
Figure 2. (a) Ball-and-stick representation of POP-CD. Color
code: Pd, blue; P, pink; K, turquoise; O, red; C, green. (b) A
schematic representation of the (γ-CD)
4
repeating unit.
In light of the above-mentioned, it is believed that, the
structural modulation of POM-CD-MOFs is accessible simply
via pairing of diverse CD and POM MBBs. More importantly,
within the rich arsenal of POMs, such hybrid assemblies can
be designed with customized functionalities. In this work, by
means of the reversible multielectron redox behavior and
electron storage functions of POTs, the electrochemical
performance of POT-CD as anode material in LIB was
evaluated. The charge/discharge voltage profiles of the POT-
CD anode for different cycles are displayed in Figure 3a. The
first discharge and charge capacities of 532 mAh/g and 132
mAh/g are obtained, respectively, with an initial coulombic
efficiency (CE) of 25 %. The irreversible capacity loss results
principally from the formation of a solid electrolyte
interphase (SEI) film due to electrolyte decomposition. The
cyclic voltammetry (CV) curves exhibit a broad cathodic
peak at about 0.5 V in the first cycle, which could be
associated with the SEI formation (Figure S9).
17
In the
subsequent cycles, two indistinct peaks became visible at a
potential of 0.9 V and 1.1 V, signifying an electrochemical
process involving the reduction and oxidation of W in PW
12
.
Meanwhile, the redox reaction of W pre/post cycling was
verified unambiguously using XPS results (Figure S10). To
further assess the battery performance, cycling stability test
was carried out. As illustrated in Figure 3b, after the first few
cycles, the CE of POT-CD quickly reached a plateau at ≈ 99%.
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Surprisingly, in stark contrast to typical POMOF anode
materials
18
, whose capacities gradually degrade, the cycling
capacity of POT-CD dramatically rose up as a function of
cycle numbers. Detailed inspection shows that a sharp
increase in capacity (up to 577 mAh/g) occurs during the first
ca. 150 cycles, which is substantially higher than that of the
first cycle. The cycling capacity steadily reached 695 mAh/g
at the 450
th
cycle, and basically stabilized at this value for
another 20 runs afterwards (Figure S11). Notably, the capacity
retention achieved an astonishing 482.6% by comparing the
capacity of the 450
th
with the 2
nd
cycle, indicating an unusual
enhanced Li storage behavior.
To unveil this nontraditional phenomenon, a combination
of PXRD and SEM characterizations has been utilized to
monitor the POT-CD anode material before and after the
cycling process. Despite the high stability of POT-CD in the
electrolyte (Figure S12), its PXRD pattern displayed no sharp
peaks after the 1
st
discharge-charge cycle, indicating an
amorphous state (a-POT-CD, Figure S13). At the same time,
a stepwise pulverization process of the MOF-derived a-POT-
CD has been observed by SEM. Before the cycling test, the
crystals of POT-CD after grinding (diameter of ca. 10-15 μm)
have been uniformly distributed on the surface of the anode
(Figure S14). Within the first 150 cycles, the size of the a-
POT-CD decreased dramatically from the order of micron to
nanosized particles with diameter of ca. 250 nm (Figure S15a-
e). Such a powdery degradation gradually slowed down, and
the morphology of the nanoparticles remained nearly
constant over 150
th
cycles (Figure S15f). Based on the above,
the upswing of cycling capacity could be assigned to a unique
“amorphization & pulverization” process: i) because of the
relatively weak linkages (e.g., K/Na···O bonds) between
MBBs in POT-CD, the high lithiation process may proceed
through Li coordination by occupying the rich binding sites
donated by CDs (hydroxy groups) and POTs (surface
oxygens). The bond cleavage of the original framework
would lead to the collapse of the skeleton and transformation
to the a-POT-CD, of which the amorphous state is capable of
providing more cation/anion vacancies, void spaces, cluster
gaps or interstitial sites for Li storage.
19
ii) due to the
electrochemical milling effect,
20
the active material (a-POT-
CD) could become smaller (pulverization) along with cycle
numbers. Consequently, more surface-active sites would be
exposed for Li intercalation/ deintercalation. The
change of morphology is in good agreement with the
obtained cycling capacity, which significantly increased (2
nd
to 150
th
cycles) accompanied by a drastic pulverization, and
slowly reached a steady state as the size of the nanoparticles
remained stable. Furthermore, as a control experiment, the
insoluble (NBu
4
)
3
[PW
12
O
40
], α-CD, and their manually mixed
analogue (PW
12
: α-CD = 1 : 1) have been prepared as
referenced anodes, of which the capacities achieved only 35
mAh/g (100 mA/g), 45 mAh/g (100 mA/g), and 40 mAh/g
(200 mA/g), respectively (Figures S16-S18). It could be
concluded that POT-CD is indispensable for the
“amorphization & pulverization” process and the exceptional
Li storage function. Additionally, an electrochemical study of
the POP-CD anode revealed a limited capacity of 85 mAh/g
(100 mA/g), which could be ascribed to the irreversible
reduction of POPs into the Pd
0
film (Figures S19).
15a
The cycling and rate performance of the POT-CD anode
(after cycling test) were examined. Reversible capacities of
688 mAh/g (at 100 mA/g) and 360 mAh/g (at 1 A/g) were
obtained (Figure 4a). When the current density was switched
back to 100 mA/g after 25 cycles, the capacities were perfectly
restored to the original state, suggesting a good reversibility
of the POT-CD electrode. Moreover, electrochemical
impedance spectroscopy (EIS) was conducted to determine
the internal resistance of the test battery. Both Nyquist plots
of POT-CD show a typical semicircle derived from the
charge transfer impedance through the electrode/electrolyte
interface (Figure 4b). The charge transfer resistance drops
drastically from ca. 280 Ω in the first cycle to 38 Ω after 300
discharge-charge cycles, which suggests an improvement of
the conductive framework in the anode during discharge-
charge process and act in concert with the cycling capacity
results.
Figure 3. The POT-CD anode: (a) Charge-discharge curves
for different cycles at 100 mA/g. (b) Cycling performance and
coulombic efficiencies at 100 mA/g.
Figure 4. (a) Rate performance at current densities from 100
mA/g to 1 A/g. (b) Nyquist plots at the open circuit potential
after the 1
st
and 300
th
discharge-charge process. (c) CV
profiles recorded at various scan rates from 0.1 to 0.5 mV s
−1
.
(d) The logarithmic relationship between currents and scan
rates for the POT-CD anode.
To further understand the charge storage mechanisms of
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